1. Field of the Invention
The present invention generally relates to noise elimination methods and transmission circuits for eliminating a crosstalk noise which is generated among a plurality of signal lines, and more particularly to a noise elimination method and a transmission circuit which eliminate a far-end crosstalk noise of a bus transmission by inserting a terminating resistor which has a specific value at a far-end.
2. Description of the Related Art
In electronic equipments such as personal computers, signal transmissions among LSI circuits in most cases are made in units of 32 bits or 64 bits. In such signals, a plurality of bits make transitions at the same timing, thereby causing signal interference among the bits and in many cases generating the crosstalk noise. The value of this crosstalk noise becomes larger as the number of signals which make the transitions simultaneously becomes larger. In addition, the crosstalk noise becomes a large value even in the case of a short line as the signal rise/fall time becomes shorter.
FIGS. 1A and 1B are diagrams for explaining a backward near-end crosstalk for explaining a background of the present invention. FIG. 1A shows a driving line 80, a driver 81, a receiver 82, a passive line 90, a driver 91, and a 35 receiver 92. FIG. 1B additionally shows an internal resistance 83 of the driver 81, a terminating resistor 84, an internal resistance 93 of the driver 91, and a terminating resistor 94 for the case shown in FIG. 1A.
In a case where two lines on which the signal transmitting directions are opposite to each other as shown in FIG. 1A, the backward near-end crosstalk refers to the noise which is introduced on the passive line 90 near the driver 81 due to the signal on the driving line 80.
FIG. 2 is a diagram showing the magnitude of the backward near-end crosstalk which is introduced in the transmission circuit shown in FIGS. 1A and 1B. In FIG. 2, it is assumed that an internal resistance 83 of the driver 81 has a value r=10xcexa9, and a resistance R of the terminating resistor 84 is infinitely large. In FIG. 2, the ordinate indicates the magnitude of the voltage, and the abscissa indicates the time. In FIG. 2. a thin solid line v1(near) indicates a voltage change on the driving line 80 on the side of the driver 81 (near-end), a thin dotted line v1(far) indicates a voltage change on the driving line 80 on the side of the receiver 82 (far-end), a bold solid line v2(near) indicates a voltage change on the passive line 90 on the side of the driver 91 (near-end), and a bold dotted line v2(far) indicates a voltage change on the passive line 90 on the side of the receiver 92 (far-end).
The backward near-end crosstalk becomes a considerably large value when the value r of the internal resistance 83 of the driver 81 is sufficiently small compared to the characteristic impedance of the passive line 90. For this reason, the value r of the internal resistance 83 is conventionally set large so as to eliminate the backward near-end crosstalk noise.
The terminating resistor 94 is provided to make a waveform matching with respect to the output signal, and the resistance of this terminating resistor 94 is set to a value approximately equal to the characteristic impedance of the passive line 90.
In other words, in a case where the characteristic impedance of the line is 50xcexa9, the terminating resistor 94 is set to approximately 50xcexa9.
Conventionally, when signals are transmitted on a plurality of lines in the same direction, no measures were taken with respect to the noise generated at the far-end on the opposite end from the driving side (hereinafter referred to as a forward far-end crosstalk noise) because the amplitude (voltage) of the forward far-end crosstalk noise is small compared to the backward near-end crosstalk noise and the effects of the forward far-end crosstalk noise with respect to the transmission line are small.
Although no measures are conventionally take with respect to the forward far-end crosstalk noise, there is a tendency for the physical distance among the signals to become smaller, due to the increased operation speed of the circuits and the reduced size and weight of the equipments. As a result, there is a tendency for the crosstalk noise to be generated more easily. More particularly, when making a parallel signal transmission of multiple bits such as 32 bits or 64 bits, there exists a case where the logic amplitude changes from a xe2x80x9c0xe2x80x9d state to a xe2x80x9c1xe2x80x9d state in all of the bits with the exception of one bit, and in such a case, the effects of the lines on which the logic amplitude of the bits which changed to the xe2x80x9c1xe2x80x9d state appear at the far-end of the signal line on which the logic amplitude of the bit remained at the xe2x80x9c0xe2x80x9d state. In some cases, such effects appearing at the far-end become large and no longer negligible. In order to simultaneously achieve the increased operation speed and reduced size and weight of the equipment, it is an object to overcome this crosstalk noise from the point of view of electronic packaging.
But conventionally, in order to reduce the crosstalk noise described above, it was either necessary to increase the physical distance among the signals or to reduce the number of signals which make the transition simultaneously. For this reason, it was either necessary to sacrifice the packaging or mounting density or to sacrifice the performance by relaxing the signal timings.
Accordingly, it is a general object of the present invention to provide a novel and useful noise elimination method and transmission circuit, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a noise elimination method and a transmission circuit which can eliminate a far-end crosstalk of a bus transmission when transmitting signals in the same direction, by a simple means.
Still another object of the present invention is to provide a noise elimination method characterized in that when transmitting signals in the same direction on at least two distributed constant lines, a resistance of a terminating resistor at a far-end is set so that voltages propagated to the far-end become equal between two kinds of propagation modes on coupled distributed constant lines, where the two kinds of propagation modes are a common mode which propagates with respect to a ground plane and a differential mode which propagates between the coupled lines. According to the present invention, it is possible to effectively eliminate the forward far-end crosstalk noise by use of a simple construction.
A further object of the present invention is to provide a noise elimination method characterized in that when first and second driving sources are coupled to respective ends of at least two distributed constant lines on which signals can be transmitted two ways, and a signal is to be transmitted from the first driving source to the other end or from the second driving source to the other end, a resistance of a terminating resistor is set so that an approximately reciprocal relationship exists between an internal resistance of the first or second driving source normalized by a characteristic impedance of the line, and a terminating resistance at a far-end with respect to the first or second driving source normalized by the characteristic impedance of the line. According to the present invention, it is possible to effectively eliminate the forward far-end crosstalk noise by use of a simple construction.
Another object of the present invention is to provide a transmission circuit having at least two distributed constant lines for transmitting signals in the same direction, characterized in that a terminating resistor is coupled at a far-end of the distributed constant lines, and the terminating resistor has a terminating resistance which is set so that an approximately reciprocal relationship exists between the terminating resistance which is normalized by a characteristic impedance of the line and an internal resistance of a driving source which is normalized by the characteristic impedance of the line. According to the present invention, it is possible to effectively eliminate the forward far-end crosstalk noise by use of a simple construction.
Still another object of the present invention is to provide a transmission circuit having at least two distributed constant lines for transmitting signals two ways, and driving sources of the signals on both end of the lines, characterized in that a terminating resistor is coupled to a far-end of the distributed constant lines with respect to each driving source, and the terminating resistor has a terminating resistance which is set so that an approximately reciprocal relationship exists between the terminating resistance which is normalized by a characteristic impedance of the line and an internal resistance of the driving source which is normalized by the characteristic impedance of the line. According to the present invention, it is possible to effectively eliminate the forward far-end crosstalk noise by use of a simple construction.
A further object of the present invention is to provide a transmission circuit coupled to at least two distributed constant lines for transmitting signals in the same direction, characterized by a terminating resistor coupled to a far-end of the distributed constant lines to reduce a far-end crosstalk noise. According to the present invention, it is possible to effectively eliminate the forward far-end crosstalk noise by use of a simple construction.
Another object of the present invention is to provide a transmission circuit coupled to at least two distributed constant lines for transmitting signals in the same direction, characterized by a terminating resistor having a resistance which makes voltages propagated on the distributed constant lines equal between a common mode and a differential mode. According to the present invention, it is possible to effectively eliminate the forward far-end crosstalk noise by use of a simple construction.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.